1. Field of the Invention
The present invention relates generally to an optical printing head for an optical printing system using an array of light emitting diodes (referred to as "LED" hereinafter), and more particularly, to a structure of an optical printing head which can be made small in scale and can be assembled with more ease without deteriorating printing quality.
2. Description of the Background Art
In recent years, optical printing systems using a combination of a small light emission point and a photoreceptor have been developed for use in terminal devices for computers, regular paper copying machines, image storing and printing devices and the like because of their advantages of high speed and noiseless operation, high resolution, and high printing quality operation. These optical printing systems are referred to as laser printers, LED printers and the like depending on the type of light source used therein.
One of the optical printing systems using an optical printing head having light emitting areas corresponding to printing dots is disclosed, for example, in U.S. Pat. No. 4,318,597. With reference to FIG. 1, in the optical printing system disclosed in the U.S. Patent with LED arrays 1 disposed alternately in two rows in a staggered configuration, data generated by a control unit 3 is transmitted in series to a shift register 4, thereby causing a LED driver 5 to drive one row of light emitting diode arrays by delaying the same by a memory, to perform predetermined printing.
Operations of a mechanical portion of this optical printing system are as follows. A photosensitive surface 7a of a photosensitive drum 7 to be driven to rotate by a motor 6 is first charged by a corona charger 8 before exposure. Thereafter, photosensitive surface 7a is exposed by LED array 2 and a short focus lens array 9 (hereinafter referred to as a "lens array"). As a result, the electric charges at the exposed portion of photosensitive surface 7a are neutralized, so that out of toner 11 applied on photosensitive surface 7a in a developing unit 10, the toner only on the exposed portion is transferred onto a sheet of printing paper 13 at a transfer stage 12. The toner 11 left on the non-exposed portion of photosensitive surface 7a is removed at a cleaning stage 14. A perspective schematic type arrangement of photosensitive drum 7, LED array 2 and lens array 9 is as shown in FIG. 2.
Relative positioning of LED array 2, lens array 9 and photosensitive drum 7 is disclosed in Japanese Patent Laying-Open No. 59-170816 and Japanese Patent Laying-Open No. 62-282957. In the positioning disclosed in these documents, the temperatures of the LED arrays and driving elements therefor increase by controlling lighting. In addition, the depth of focus of the lens array is small and the distance between a light source and a photosensitive surface (i.e. an object-image surface distance) should be precisely set within a conjugation length, with a margin as small as around .+-.0.2 mm. In particular, such variation caused after positioning should be prevented as variation in an optical position caused by a board curve due to the increased temperature thereof resulted from driving of a LED array.
A means for preventing this variation in optical position is disclosed, for example, in U.S. Pat. No. 4,733,127. As shown in FIG. 3, the device disclosed in this U.S. Patent is provided with a flat radiator plate 15 on which a board 1 with a LED array 2 attached thereto is fixed by means of an adhesive material or the like, and a lens holder 16 for holding a lens array 9 is fixed by means of, for example, screws 17, thereby preventing effects of thermal deformation to maintain a once adjusted optical distance. In addition, a transparent glass plate 19 supported by protection frames 18 covers over LED array 2.
As shown in FIG. 4, lens array 9 is structured, for example, by optical fiber lens 9a sandwiched by sandwich plates 9b. The refractive index of optical fiber lens 9a is at a maximum at the axial center thereof and decreases approximately directly as the square of a radius from the axial center. The optical fiber lens therefore serves as a convergent lens even if a light receiving and emitting plane is flat, thereby converging the light emitted from LED array 2 positioned at a distance of the focal length f from the light receiving end surface of lens array 9 onto a photosensitive surface 7a positioned at a distance of the focal length f from the light emitting end surface of lens array 9 as shown in FIG. 5.
A converging rod lens array consisting of, e.g. the SELFOC lenses can be used in place of lens array 9 including optical fiber lens 9a. The ELFOC lenses have a radial distribution of refractive indexes from its center toward its periphery to allow light to pass therethrough in a zigzag direction in a fixed cycle, thereby functioning as image forming lenses.
However, the structure of the optical printing head shown in FIG. 4 requires a radiator plate 15 having a large area for fixing board 1 and lens holder 16 thereon, which is followed by an increase of a longitudinal width of the optical printing head. Taking into consideration that many parts such as corona charger 8 and developing unit 10 are arranged on the periphery of photosensitive drum 7 of the optical printing system to which the optical printing head is attached as shown in FIG. 1, the increased width of the optical printing head prevents a reduction in scale of the system.
In addition, in order to securely fix board 1 and lens holder 16 on radiator plate 15 for keeping the optical distance constant, many screws 17 and other fixing members are required. With many fixing members required, fixing the members for adjusting the above-described optical distance is very complicated, and moving the fixing members when the optical printing head is fixed results in a displacement of relative position of the adjusted optical elements.